A resin composition containing a block copolymer of a vinyl aromatic hydrocarbon and a conjugated diene with a relatively high content of the vinyl aromatic hydrocarbon, especially a resin composition having a block copolymer of styrene and butadiene blended with a styrene polymer (GPPS) is widely used for injection molding or for extrusion to obtain a sheet, a film, etc., since its formed product has properties such as high transparency and impact resistance. And, various proposals have been made for the purpose of further improving properties of a formed product obtainable from such a resin composition containing a block copolymer, such as rigidity, surface hardness, folding strength, elastic modulus in tension, sheet impact, transparency and impact resistance.
For example, a polystyrene polymer composition having a structural formula (A-B)—Si comprising a styrene/butadiene copolymer block A containing at least 50 wt % of styrene and a styrene/butadiene copolymer block B containing at least 50 wt % of butadiene, is disclosed in Patent Document 1. Further, a method for producing a block copolymer, for producing a block copolymer comprising a conjugated diene and a vinyl-substituted aromatic hydrocarbon by a batch polymerization method using an organic lithium compound as an initiator, wherein (a) the organic lithium compound and a monomer constituting a first stage block are continuously supplied simultaneously, and (b) the supply time ratio of the organic lithium compound/the monomer constituting the first stage block is from 0.1 to 1.0, is disclosed in Patent Document 2.
Further, a block copolymer composition comprising two types of block copolymers each having a specific content of a vinyl aromatic hydrocarbon constituting the block copolymer and a specific block ratio of the vinyl aromatic hydrocarbon polymer block, and a styrene resin in combination, is disclosed in Patent Document 3. A styrene resin composition comprising a linear block copolymer of which the weight ratio of a vinyl aromatic hydrocarbon to a conjugated diene, a polymer block between a terminal polymer block and a terminal polymer block, the molecular weight of the polymer block, and the molecular weight of a vinyl aromatic hydrocarbon polymer block are controlled, is disclosed in Patent Document 4. Further, a block copolymer which comprises a polymer block containing a vinyl aromatic hydrocarbon as the main component and a polymer block containing a conjugated diene as the main component, which has a polymer structure of a specific linear block copolymer, in which the constituting monomer units are in a specific mass ratio, and which has a specific molecular weight distribution, is disclosed in Patent Document 5.
Further, Patent Document 6 discloses a block copolymer of which a specific local maximum value of the loss elastic modulus is observed within a temperature range of from −20° C. to +20° C. by dynamic viscoelasticity measurement, and of which the mass ratio of the conjugated diene based on the total mass of the block copolymer is within a specific range, focusing attention on the dynamic viscoelasticity of a block copolymer containing a vinyl aromatic hydrocarbon and a conjugated diene not disclosed in the above Patent Documents 1 to 5.
However, with respect to the above conventional block copolymer resin compositions and resin compositions of such compositions with various thermoplastic resins, it is desired to achieve more excellent properties of a formed product obtainable from the resin compositions, especially a sheet, a film, etc., that is, higher impact resistance, transparency, etc., without decreasing properties such as the sheet impact, folding strength and elastic modulus in tension.